Microcomputers are sophisticated, general purpose logic devices which can be programmed to perform a wide variety of useful control functions in industrial and communications equipment, large-scale and medium-scale computer peripheral and terminal hardware, automobiles and other transportation media, amusement and educational devices, and the like. Generally, an entire spectrum of microcomputers is presently available in the commercial marketplace. As the speed of operation, or throughput, of a microcomputer increases, the more valuable and more versatile the microcomputer becomes, since it is able to control a given operation more efficiently and more accurately, or to control a greater number of operations simultaneously, or to control operations requiring relatively fast response times.
In a single-chip microcomputer having serial I/O communication logic it is desirable to minimize the number of unnecessary interruptions of the on-board microprocessor or CPU. By reducing unnecessary interrupts of the CPU, the efficiency of the microcomputer and the throughput of the CPU are increased. It is becoming increasingly common to connect several microcomputers together in what is known as a distributed processor or multi-processor configuration, wherein the microcomputers may share a common serial I/O communication line. When serial communications are being conducted between a master CPU and a slave CPU, certain digital information transmitted over the shared serial I/O line may not be of interest to all of the CPUs. It is desirable to provide a means for a CPU to selectively ignore the contents of a message which is not specifically addressed to it and to "wake-up" when the serial I/O line again becomes free, in order that it may receive any future digital communications of interest to it.